(1) Field of the Invention
The present invention relates generally to synchro-servo system control and more particularly to an analog, "infinite" resolution, synchro control differential transmitter, where "infinite" resolution connotes theoretically infinite resolution (versus digitally finite resolution) because resolution is always, as a practical matter, limited by various noise mechanisms.
(2) Description of the Prior Art
It is well known that synchro-servo control systems have been used for many years for remote positioning of mechanical devices. FIGS. 1A and B depict the symbol and schematic diagram respectively of a synchro control transmitter, referred to hereinafter as a "CX". An input AC voltage is applied across the rotor terminals R1 and R2 as V.sub.R1-R2 which is defined as, EQU V.sub.R1-R2 =K.sub.R1-R2 SIN .omega.t (1)
where K.sub.R1-R2 represents an input root mean square (RMS) voltage and .omega.=2.pi.f, f being the input voltage frequency which is typically 60 Hz or 400 Hz. The output voltages appear across stator terminals S1, S2, and S3 as V.sub.S1-S3, V.sub.S2-S3 and V.sub.S1-S2. These synchro output voltages, depicted by `.theta.`, are functions of shaft angle .theta., i.e., the relative angle between the rotor and stator, and are defined as; EQU V.sub.S1-S3 =K.sub.S1-S3 SIN .omega.t (2) EQU V.sub.S2-S3 =K.sub.S2-S3 SIN .omega.t (3) EQU V.sub.S1-S2 =K.sub.S1-S2 SlN .omega.t (4)
where EQU K.sub.S1-S3 =R[K.sub.R1-R2 SIN .theta.] (5) EQU K.sub.S2-S3 =R[K.sub.R1-R2 SIN (.theta.+120)] (6) EQU K.sub.S1-S2 =R[K.sub.R1-R2 SIN (.theta.+240)]. (7)
Note that an angular signal in ` `, as used in FIG. 1 et seq., indicates a set of synchro or resolver voltages corresponding to that angular signal.
The transformation ratio "R" of output-to-input, graphically illustrated in FIG. 2, is the maximum line-to-line output RMS voltage, i.e., for a particular CX, the maximum RMS voltage across any two stator terminals divided by the specified constant input RMS voltage, EQU R=[K.sub.S1-S3 (max)]+[K.sub.R1-R2 ] (8)
Typically, the input voltage to synchro devices is 115 V.sub.RMS, while the maximum output voltage is 90 volts line-to-line RMS, V.sub.L-L RMS. Thus, EQU R=90.div.115=0.7826. (9)
FIG. 3 depicts the symbol for a synchro control transformer, or a "CT". It's output voltage, available across rotor terminals R1 and R2 as V.sub.R1-R2, is a function of the input synchro voltages, which correspond to an angle .theta., and the shaft angle .theta., where, EQU V.sub.R1-R2 =K SIN (.phi.-.theta.), (10)
and where, EQU K=K.sub.o SIN .omega.t, (11)
K.sub.o being a constant which includes R and the input maximum line-to-line RMS voltage.
FIG. 4 shows a typical system. Excitation is applied to the CX input and the output is an AC voltage whose amplitude is proportional to the SINE of the difference between the CX shaft angle .phi. and the CT shaft angle .theta., EQU V.sub.R1-R2 =K SIN (.phi.-.theta.). (12)
FIG. 5 illustrates the symbol for a synchro control differential transmitter, or a "CDX". Its output voltages, available across rotor terminals R1, R2, and R3, are a function of the input synchro voltages, which correspond to an input angle .phi., and the shaft angle .theta., such that the output corresponds to the angular difference .phi.-.theta..
FIG. 6 demonstrates how a CDX is employed in a typical system. Excitation is applied to the CX and the output is an AC voltage whose amplitude is proportional to the SINE of the CX shaft angle .phi. minus the CDX shaft angle .theta., and minus the CT shaft angle .beta., EQU V.sub.R1-R2 =K SIN (.phi.-.theta.-.beta.). (13)
By crossing a given pair of stator leads or rotor leads, the output can be made a function of any desired sum or difference of the input angles. One example of this is given in FIG. 7. By starting with the system of FIG. 6 but connecting the S1 terminal of the CX to the S3 terminal of the CDX and connecting the S3 terminal of the CX to the S1 terminal of the CDX, the output is changed from corresponding to .phi.-.theta.-.beta. to corresponding to -(.phi.+.theta.+.beta.).
An electronic CDX, or an "ECDX", is a CDX whose "shaft angle" input is not an actual mechanical shaft, but a voltage level scaled, usually but not necessarily, linearly to a fictitious "shaft angle." It is understood that with all the electronic control synchro-servo systems in use or under development, it is not always convenient to have to drive an actual mechanical shaft. Many times the control signal is already in voltage form and to convert that voltage to drive the shaft of an actual electro-mechanical CDX, as illustrated in FIG. 8, has many disadvantages. The first disadvantage is that the input voltage cannot be infinite. An actual synchro shaft, however, can be turned an infinite number of degrees, i.e., continuous rotation. This is an obvious and trivial disadvantage with all known electronic CDX's as many CDX applications do not require continuous shaft rotation. Other disadvantages of the FIG. 8 implementation are that it; requires moving parts, is a large and bulky design, has a high parts count, exhibits servo feedback loop instability, requires fine mechanical precision, and is mechanically complex. The one advantage of this implementation, however, is "infinite" resolution.
FIG. 9 illustrates another common implementation, i.e., an analog/digital electronic CDX. A digital, solid-state CDX such as that shown is commercially available. The A/D converter converts an input analog voltage to a digital word which corresponds to the shaft angle. The advantages of this technique are that it; requires no moving parts, is of reduced size, and is very stable due to the absence of feedback. Its disadvantage is finite resolution due to digitization.